1. Field of the Invention
This invention relates to light biasing of vidicons, and in particular to the optimization of light biasing to improve vidicon performance.
2. Description of the Prior Art
Vidicons are popular camera tubes because of their ability to operate at low light levels and their ability to rapidly respond to changes in the intensity of light from an object to be imaged. The response time is limited, however, by a phenomenon known as lag. There is both a build-up lag, which is a delay in responding to instantaneous changes from lower intensity to higher intensity light, and a decay lag, which is a delay in responding to instantaneous changes from higher intensity to lower intensity light.
As is discussed in more detail in U.S. Pat. No. 4,359,759 to McBride et al., which is hereby incorporated by reference, the adverse effects of lag are most significant at low levels of incident light. At levels slightly above the dark (no light) level, an RC time constant and electron traps associated with the vidicon electron beam and its photoconductive target combine to cause an exponential delay in charging and discharging of different areas of the target in response to light intensity changes at corresponding areas of the image focused onto the target. When the vidicon is used to produce a series of video image frames in rapid succession, these delays may cause wasted frames. This typically occurs at start-up, when the intensity of image light focused onto the target suddenly increases from the dark level to a pattern of light levels corresponding to the image. Similarly, wasted frames may be produced whenever the level of light received from any significant area of the image suddenly decreases to the dark level. Wasted frames might be tolerable in some applications, but not in applications such as medical radiography where the consequence is an increase in the radiation dosage received by a patient. In such applications a patient is irradiated with x-rays during a period in which the vidicon produces a series of video image frames of a fluoroscopically-produced optical image of the x-ray radiation passing through the patient. If a number of frames are wasted as the electron charge levels on different areas of the vidicon target increase from a zero charge level to charge levels representative of the optical image focused thereon, the number of frames during which the patient is irradiated must be increased by a corresponding number.
To reduce these charging delays, vidicons are often provided with bias lighting arrangements for uniformly illuminating their targets with low intensity light radiation. These arrangements include internal bias lighting means, such as those disclosed in U.S. Pat. No. 4,259,609 to Month et al. and in other U.S. patents referred to in column 1 of the Month et al. patent, and external bias lighting means such as that disclosed in the McBride et al. patent. By illuminating the target with low intensity bias lighting before the vidicon begins to produce video frames of an image, the target is brought to an initial charge level which reduces lag and correspondingly decreases the number of wasted frames. Unfortunately, as the intensity of the bias lighting is increased to reduce lag, the vidicon sensitivity to low image intensities is reduced. Because of this decrease in sensitivity, bias lighting means frequently are not used, even in vidicons already provided with internal bias lighting sources.
The McBride et al. patent discloses a radiographic apparatus in which the target of a vidicon is momentarily pre-illuminated by a bias lighting means disposed between the target and a lens for focusing an optical image from an x-ray image intensifier onto the target. The purpose of this pre-illumination is to shorten the transient period needed for the vidicon target to charge to a linear response region. In the preferred embodiment, the bias lighting means is an annular light diffuser containing a plurality of light emitting diodes. Light emitting diodes are chosen because they can be switched off in times which are short in comparison with typical vidicon frame periods. Timer circuitry controlling the bias lighting means effects pre-illumination of the vidicon target for a predetermined period prior to irradiating the patient with x-rays to produce the optical image. Just prior to irradiation, the timer circuitry causes the bias lighting means to decrease illumination from a high pre-illumination level to a lower level, or to zero. This decrease in illumination is apparently intended to improve the sensitivity of the light biased vidicon to low level image intensities. However, the timer-controlled bias lighting arrangement disclosed complicates the radiographic apparatus in which it is used, requires a light source having a rapid switching capability, and is not usable with certain vidicons such as those which have their targets direct-coupled to image intensifiers by fiber optics or other means.